As various different types of communication systems have arisen for radiotelephones, it has become beneficial to provide portable and mobile radiotelephone stations that are interoperable between these various communication systems. As a first step, dual-mode phones have been developed that can operate between two radiotelephone systems. For example, the Global System for Mobile (GSM) communication and the Wideband Code Division Multiple Access (WCDMA) communication systems are intended to work together in the same mobile terminal equipment operated under a Universal Mobile Telecommunications System (UMTS) environment, wherein a mobile radiotelephone is required to scan for a home Public Land Mobile Network (PLMN) and higher priority PLMNs in these two alternate radio access technologies (RAT) and all possible frequencies within each. In particular, when a UMTS radiotelephone roams and camps on a visited PLMN, the radiotelephone is required to search for its home PLMN and higher priority PLMNs in all possible frequencies of these two radio access technologies. In this way, a single mobile station radiotelephone can automatically determine the availability of its home PLMN, and thus obtain service on its home PLMN if it is available in a given location. In particular, the 3GPP specifications allow for a mobile station to perform (background) scans for PLMNs other than the one on which it has currently obtained service.
Currently, GSM and UMTS cellular phones are required to perform a periodic search for higher priority PLMN, or their home PLMN (HPLMN) whenever the radiotelephone is camped on a Visited PLMN (VPLMN) and in their home country. This search is required to be performed periodically at a rate which is specified on the Subscriber Identity Module (SIM) or Universal SIM card and is a multiple of six minutes (with the fastest rate being once every six minutes). This search consumes battery power to perform because the radiotelephone must measure power on all frequencies of all bands which it supports and then synchronize to each frequency on which there is appreciable energy and read the PLMN identification of the cell. Note that for UMTS radiotelephones which support both GSM and WCDMA Radio Access Technologies (RATs), the radiotelephone is currently required by 3GPP specifications to perform the search in both RATs (i.e. the phone must perform the search for the HPLMN in all radio access technologies of which it is capable). This is true even if the HPLMN network has cells of only one RAT. Therefore, the requirement to search for all possible frequencies in all possible RATs wastes significant battery power.
Currently for the GSM Radio Access Technology (RAT), there is an information element (IE) called Broadcast Control Channel Allocation (BA) Range which may be included in a Channel Release message sent to individual radiotelephones (i.e. on a phone-by-phone basis). This information element contains a non-inclusive list of frequencies in use by the network. Similarly, for WCDMA Radio Access Technology (RAT), there is an information element called Radio PLMN (RPLMN) Information that may be included in a Radio Resource Control (RRC) Connection Release message sent to individual radiotelephones (i.e. on a phone-by-phone basis). This information element also contains a non-inclusive list of frequencies in use by the network. Neither of these information elements are guaranteed by the specifications to be inclusive lists of absolutely all frequencies in use across an operator's entire network within a country. The specifications only allow the radiotelephone to make use of these information elements to speed up the cell selection process by searching on the frequencies specified by these information elements first, before searching on other frequencies. Specifically, the 3GPP 25.331 specification, version 3.12.0 (September 2002) in section 8.1.4.3 states, in regards to the RPLMN information element, that the radiotelephone may “. . . utilise this information, typically indicating where a number of BCCH frequency ranges of a RAT may be expected to be found, during subsequent RPLMN selections of the indicated PLMN.” Although useful to speed up scans, the specification does not state that the radiotelephone may search only on these frequencies, nor that these are the only available frequencies.
Therefore, the need exists for a method to allow a mobile unit to obtain an inclusive list of all available frequencies in all available radio access technologies. Moreover, it would be an advantage to scan for only those frequencies that are in use and are supported by the HPLMN and/or higher priority PLMNs. It would also be of benefit to provide this performance improvement with little or no additional cost.